In a power module which includes a power device formed of a semiconductor device, such as an IGBT (Insulated Gate Bipolar Transistor), the semiconductor device must be held at a predetermined temperature or lower by means of efficiently radiating heat generated therefrom. A base for a power module (hereinafter referred to as a “power module base”) which meets the requirement has conventionally been proposed (see Patent Document 1). The power module base includes a ceramic insulating substrate having an aluminum wiring layer formed on one face thereof and an aluminum heat transfer layer formed on the other face thereof; an aluminum heat radiation substrate brazed to the heat transfer layer of the insulating substrate; and an aluminum heat sink brazed to a side of the heat radiation substrate opposite the side brazed to the insulating substrate. A cooling liquid flow path is formed within the heat sink.
In the case of the power module base described in Patent Document 1, a power device is mounted on the wiring layer of the insulating substrate, and various components necessary for the power device, such as a resin casing for covering the insulating substrate and the power device, are attached onto the heat radiation substrate. Alternatively, an electrical wiring frame, electronic components, etc. are attached onto the heat radiation substrate. Thus, a power module is completed. Such a power module is applied to an inverter circuit of a movable body such as a hybrid car in which an electric motor is used as a portion of a drive source, and the power module controls electric power supplied to the motor in accordance with the operating state of the movable body.
Heat generated from the power device is transferred to the heat sink via the wiring layer, the insulating substrate, the heat transfer layer, and the heat radiation substrate, and is radiated into a cooling liquid flowing through the cooling liquid flow path.
Incidentally, in the case of the power module base described in Patent Document 1, when the insulating substrate, the heat radiation substrate, and the heat sink are heated at the time of heating for brazing, the insulating substrate, the heat radiation substrate, and the heat sink thermally expand. The insulating substrate, the heat radiation substrate, and the heat sink are brazed in a thermally expanded state, and thermally contract after completion of the heating. The heat radiation substrate and the heat sink, which are formed of aluminum whose linear expansion coefficient is relative high, tend to thermally expand by a relatively large amount upon heating at the time of brazing. Meanwhile, the linear expansion coefficient of the ceramic which constitutes the insulating substrate is lower than that of aluminum. Therefore, even when the insulating substrate is heated at the time of brazing, the insulating substrate does not thermally expand greatly, as compared with the heat radiation substrate and the heat sink. As a result, the degrees of thermal contraction of the heat radiation substrate and the heat sink become greater than that of the insulating substrate. Therefore, if no measure is taken, when the insulating substrate, the heat radiation substrate, and the heat sink thermally contract as a result of being cooled to room temperature after completion of brazing, the degrees of contraction of the heat radiation substrate and the heat sink become greater than that of the insulating substrate, and the heat radiation substrate and the heat sink are pulled by the insulating substrate, so that deformation such as warpage arises in the heat radiation substrate. As a result, it becomes impossible to accurately attach various components onto the heat radiation substrate. For example, in the case of a resin casing for covering the insulating substrate and the power device, there is a possibility that the resin casing cannot be attached in a sealed condition without forming a clearance. Further, there is a possibility that an electrical wiring frame and components necessary for the power module, such as electronic components, cannot be accurately attached.
In the case of the power module base described in Patent Document 1, the above-described warpage of the heat radiation substrate and the heat sink is suppressed by increasing the thickness of the heat radiation substrate. However, if the thickness of the heat radiation substrate is increased, the heat conduction path from the power device to the heat sink increases in length, and heat radiation performance may drop.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2003-86744